But one more year until the end of century 20.1?

century End Century by kevindooley

A little decadence
[ViaBad Astronomy ]

Quite by accident, just the other day I found myself embroiled in a controversy on Twitter of my own making. I made an offhand mention that the decade would be ending in a few short days. That seemed obvious enough to me, but apparently not so to many others. What ensued was something of a firestorm of people, many of whom disagreed with me. However, I maintain that I was right all along. Here’s the scoop.

My claim is that December 31, 2009 — today, as this is posted — is not just the last day of the year, but the last day of a decade. Now, I don’t mean that in the trivial sense that any moment is the last moment of the past ten year period — you can always talk about the last ten years that end at any time.

I meant, and still mean, specifically the first decade of the 2000s. That does in deed and in fact end today.

What people were arguing over were things like centuries and millennia, and how there was no year 0, and therefore the last day of the decade is actually December 31, 2010. But that’s not relevant because we don’t measure decades the same way we do centuries.

Certainly, the last day of the 20th century was December 31, 2000. In that case, there was no year 0, so the first year of the 1st century ended on December 31, 1 A.D. Doing the math, it’s easy to see that 1999 more years needed to elapse to end the 20th century, and so its demise was on that last calendar day of 2000. January 1, 2001 marked the first day of the 21st century.

But we don’t reckon decades like that. We refer to them by the tens place in the year’s numerals: the 70s, the 80s, the 90s. And since we do, clearly, today is the last day of the decade we will call the aughts or zeroes or whatever


I just had this exact same discussion. It is almost as if we did too good a job 10 years ago explaining that 2000 was not the beginning of a new century but that 2001 was (thus the name of the book/movie). I was on one of those Y2K committees charged with preventing Armageddon by making sure all sorts of paperwork was completed. I spent many times explaining all of this to people.

A decade has a year 0, thus ten years pass after the end of a year ending in 9. Thus today is the last day of the decade. But since we do centuries differently, I guess we can call next year the end of century 20.1!